328 Dr. Mac Culloch on the Geology of Glen Tilr. 



surface of the earth. Geologists, hereafter perhaps more attentive 

 to this subject, may probably add to this catalogue, or at least aug- 

 ment the number of examples, and add many more to the few lo- 

 calities which I have had opportunities of examining. 



To surveyors who are more deeply interested in the fact and its 

 consequences, I shall also look for more extensive and more accu- 

 rate observations, since the possession of more delicate instruments, 

 and the devotion of time and attention to this particular object, are 

 required to determine the quantity and extent of these influences, as 

 well as their practical effect in producing permanent local variations 

 in the magnetic meridian. 



I. need scarcely say that an ordinary ships compass is insufficient to 

 detect these variations, unless where they are considerable, as the 

 instrument, either from the rudeness of its workmanship, or the 

 intentional insensibility given it by the maker to render it steady in 

 steering small ships or in navigating through a cross sea, is rarely 

 alive to minute quantities of the disturbing force. 



I have attempted on various occasions to discover the positions 

 of the poles of the natural magnets which thus disturb the needle 

 brought within the sphere of their influence. From the observation 

 of Humboldt we are led to imagine that he conceived the rock or 

 mountain which he describes, to have possesed but two poles, and 

 that its effects on the needle could in consequence be easily as- 

 signed. This is not impossible, but in all the cases where I have 

 been able to make observations, I have found that the disturbances 

 must have been produced by a number of independent magnets, 

 each rock or fragment being possessed of a meridian of its own, 

 and the disturbances of the needle being in consequence of 

 that, extremely irregular and uncertain. I have already entered 

 into a detail of this fact where I first observed it in Sky, in the 



